This letter presents a 5 b phase shifter using a commercial 0.18 SOI process for X-band phased array antenna. The phase coverage of 360 with the LSB of 11.25 is achieved at 8–12 GHz. The RMS phase error and amplitude variation are less than 6.5 and 0.5 dB, respectively. The measured insertion loss is less than 10.8 dB, and the input and output return losses are over 12 dB at 8–12 GHz. The current consumption is nearly zero with 1.8 V supply. The chip size is excluding pads. To the authors' knowledge, this is the first demonstration of X-band phase shifter using a commercial SOI technology.